When you walk into a shop to buy a race-level road bike, the process might seem more complicated than ever--and not only because you have to decipher all the claims that "our carbon is better than theirs," or "this new bottom-bracket design is best," or "trust us, electric shifting is revolutionary." Choosing a new bike can be confounding because a growing number of manufacturers are producing two distinct frames that can each be called the company's fastest.

One style of bike has the telltale, angular shaping of a wind cheater; the other has the delicate curves of a lightweight that flouts gravity. And this time the hype is true: Each one really might be the fastest bike you've ever ridden--it depends on the kind of speed you crave.

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FEATHER VS. KNIFE Of the many ways to evaluate a frame's potential performance, lightness is the easiest to understand. You can simply pick up two frames and feel a weight difference (which might explain, in part, why so many cyclists obsess over every gram). Everyone intrinsically understands that we can move a lighter object more easily than a heavier object, so paying more for a lighter bike makes sense.

This is backed up by more thorough study. Unlike some commonsense ideas that turn out to be false, weight truly does matter, especially on climbs. Reliable calculators, easily found on the Internet, show how much faster a lighter bike is on an ascent. For example, on a 6-mile hill with a constant 8 percent grade and a rider who puts out a consistent 250 watts, reducing weight by 250 grams will result in a time savings of more than seven seconds. (But remember that these calculations are based on unrealistically constant parameters-- in the real world, wind changes speed and direction, the grade varies, your friends draft or attack.) To smaller degrees, a lighter bike also helps when you're quickly accelerating, such as in an attack, or changing speeds frequently to adjust for grade or your companions. By contrast, the effect of a bike's weight can be surprisingly negligible on a flat road at a constant effort.

Aerodynamic drag has significantly more impact on you--up to 90 percent of the resistance you're working against when riding a bike. Because drag is every rider's biggest foe, some companies put monumental effort into making bikes that slice through the air. But the payoff isn't always proportionate to the effort: Unlike the instant feedback a potential buyer gets by lifting a bike in a showroom, the benefit of aero improvements can be difficult to grasp. More important, so much drag comes from the rider that the bike is responsible only for about 10 percent of that 90 percent of resistance. Even so, improvements to frame aerodynamics matter. Cervelo cofounder Gerard Vrooman says that his aero-shaped S3 frame lets the average rider gain 10 meters per kilometer compared with the R3-SL , which is 200 grams lighter (again, more numbers based on controlled situations, and which may not translate to your Tuesday-evening loop).

So to recap: Improvements to either aerodynamics or weight result in additional speed. The difference is in the type of speed. In general, the aero bike is faster on flats and longer, sustained efforts, and the lighter bike is faster on climbs and in pack situations (with lots of drafting that minimizes aerodynamic advantages) that involve repeated, sharp accelerations.

When given a choice, racers at the pro level almost always opt for the aero design. This is because by rule, their bikes can weigh no less than 14.9 pounds. That limit is attainable even with a heavier aero frame. For the rest of us, the answer isn't so clear. We can make our bikes as light as we want, for one thing. For another, very few of us have one bike for the Alpe d'Huez time trial and another for the punchy rollers leading to the Massif Central. We ride the same bike on the local climbs, on the crosswind-battered club ride that always breaks up, in the weekly crit and for the flat recovery day. Cervelo took a swing at quantifying the difference between its light R3-SL and aero S3, based on an average rider, with real-world riding conditions ( including turns, stops and group riding) using real roads (the route of the 2007 L'Etape Du Tour, a 120-mile ride with more than 15,000 feet of climbing). Winner: the S3, 200 grams heavier but 1.5 percent more aerodynamic, by less than a minute.

"For top-level riders, such small differences can make big impacts," says Vrooman. In this year's Tour de France, for instance, the top 10 riders were separated by just less than eight minutes, or about 0.17 percent of their total ride time. That's reason enough for a company like Scott to develop its Project F01, which debuted at this year's Tour. The manufacturer's marketing director, Adrian Montgomery, says the still-prototype frame weighs 840 grams, 50 more than the company's flyweight Addict frame, yet offers a 20 percent reduction in frame drag. That means a rider can maintain 28 mph on a flat road using 4 to 5 percent fewer watts.

For mere mortals, however, the differences might seem laughable. Sure, the R3-SL weighs 200 grams less, and you can feel that if you lift the bikes one after the other. "But think of 200 grams in terms of a total rider-plus-bike weight of 70 to 80kg, and it becomes a different story," says Vrooman. "That's 0.25 percent weight savings. Is that really something to become excited about?" And sure, the S3 is 1.5 percent more aero, but the advantage it gives you, even on a tougher day than many of us will ever do, adds up to fewer than 60 seconds. Sit up too long to chug from your water bottle and you could negate that.

WHERE RUBBER MEETS ROAD We put the conundrum to the real-world riding test by pitting Blue's 2011 ultralight Axino against its aerolight AC 1 SL , which we reviewed last April.

Our size-medium AC 1 test model weighed 13.09 pounds (without pedals or cages). Of the many ways the frame is shaped to maximize aerodynamics, the bulging and swept-back shapes of the fork, head tube and down tube are most noticeable, as well as the seat-tube cutout to hold the rear wheel. (The down tube also has a cutout for the front wheel.) In our initial test, we raved about the bike, lauding the ride as "quick under power" and "fast under sustained output." We kept the bike for extended testing under one of our sprint-specialist test riders, a pro track racer. It became his go-to bike for everything from crits to allday training rides (with the Zipp 404 tubular wheels swapped out for clinchers). "When the sprint is starting and you come out of the draft and start pulling past people for the line," he says, "the bike just slices through the massive headwind you run into. And it just keeps getting faster all the way. It doesn't so much surge as gain speed as it goes." In his experience, the longer the wind-up and sprint, the better the AC1 performed. We also found that the bike almost seemed to encourage long, solo breakaways when you hunker down and pour power into the pedals; similarly, we had some of our most fun moments on the bike while speeding along on routes with long, slight drops in elevation.

The Axino has nearly identical frame geometry but shaves 150 grams. The bike is 30 percent stiffer than the AC1 in the head tube and bottom bracket, according to Blue's product development manager, Chris Pic, which is unusual for a lighter frame when compared with its aero kin.

Whether it's the slightly lighter frame or the additional stiffness or both, the Axino feels jumpier than the AC1. This is the bike for the rider who likes to stand up on a steep climb, attack, ease off the pedals to see who can catch up, then leap away again. And again and again. The Axino also feels fast in aggressive and even sketchy group situations--its propensity to respond just ever so slightly quicker than other bikes seems nearly telepathic, or as if for an instant you can see into the future and react before everyone else. In a sprint, you want to wait and wait and wait until winning is nearly impossible, then come around in one burst.

Not surprisingly, our sprint specialist preferred the AC1. The Axino was favored by our steep-hill lovers, by those who love to play--repeatedly attacking the lunch ride, for instance--and by one tester who likes to sit in the group until pack dynamics lead to a situation in which a sharp effort can spring breakaways or, at least, pop the inattentive or tired off the back. What was surprising? Our sprinter complained about the Axino's stiffness. "It feels stouter than the AC1, but not in a way I like," he said. "I feel more input from the road but without a higher top speed--so for me there's no upside."

We're drawing ultrafine distinctions here. Either would make an excellent all-around bike if you were doomed to own just one. And that's the best thing about this aero-versus-light riddle: You can make such distinctions, but you can't make a bad choice.

QUICK CHOICES
HERE'S A ROUNDUP OF SIX MORE LIGHT/AERO PAIRINGS TO CONSIDER.